To land an aircraft the pilot descends at a uniform slope, about 3 degrees, on approach to the landing area. When over the landing surface he executes a maneuver called "flare" or "flare out". During the "flare" he lifts the nose of the aircraft to arrest the previous sink rate and positions the aircraft in the landing attitude which is slightly nose high. The "flare" is executed ideally at a particular height above the landing surface, with the "flare" height being a function of the aircraft type, on the order of 1/2 to 1 wing span. If the "flare" is executed at too high a height above the landing surface, there exists a high probability of wing stall due to the fact that the nose high attitude is causing a rapid decrease in air speed. Such a stall would result in either a very hard landing, probably followed by bounce, or a catastrophic crash onto the landing surface. If the "flare" is executed at too low a height the aircraft will be in a nose low attitude which could result in a propeller strike in single engine aircraft, or a nose wheel first touch down which invites "wheel barreling" or "ground looping", both of which produce an uncontrollable direction of travel and generally result in substantial damage and/or injury.
A primary requirement during the landing phase of flight is therefore to have a knowledge of height above the landing surface while in the region of tens of feet in order to execute the "flare". A secondary requirement is to have a knowledge of height when in the region of a few feet to less than one foot so that up elevator can be applied to cause a smooth transition, near a zero rate of decent, from flight to rolling on the tires.
All aircraft determine in-flight altitude with a barometric altimeter referenced to Mean Sea Level (MSL). The barometric altimeter is never used by the pilot during the landing phase below about 200 feet above ground level (AGL). This is because current knowledge of the barometric pressure at the landing site cannot be trusted to the accuracy required in that 0.01 inches Hg corresponds to about 10 feet; and, the FAA required accuracy for such an altimeter is not sufficient for the decisions that have to be made during the final landing stage. Air Transport (ATP) category aircraft determine altitude during the landing phase by means of a radio altimeter. The radio altimeter echo ranges off the ground with a continuous wave or pulsed microwave carrier signal in a manner analogous to radar. The radio altimeter is in fact sometimes referred to as a radar altimeter. However, General Aviation aircraft are generally precluded from being equipped with a radio or radar altimeter for monetary reasons. This is unfortunate since the vast majority of the operational aircraft in the country, and the world, are General Aviation as opposed to air transport category aircraft. As a result of this, General Aviation pilots determine altitude or height above the landing surface by visual observation. And they do so day and night, even during instrument landing approaches; and over water for amphibious aircraft.
The problem with this is that a person cannot determine, even approximately, their height above a flat surface runway by looking down at it. For this reason student pilots are taught to look straight ahead and not down the runway. What the pilot is doing, without realizing it, is waiting for the runway edges to come up in his or her peripheral vision to an angle from the horizontal that corresponds to that used as a "flare" point in previously successful landings. This is why private pilots, in particular, are always doing "touch and go" practice landings.
Another problem that leads to "flare" point problems is variation in runway width, which is why pilots tend to "flare" high on wide and low on narrow runways. Since runway widths can vary from about 30 feet to over 200 feet this presents a significant problem to many pilots and explains why landings at unfamiliar airports are usually not as good as at their home base.
At night even a familiar airport induces a high "flare" point in that the runway edges appear to be further apart due to the indistinct contrast between pavement and ground. On the other hand, bright runway edge lights could produce the opposite illusion.
Sea plane pilots have an extreme problem in estimating their height above the water surface on which they intend to land. The visual clues used here are not well understood, except that it is well known that a choppy to slightly ruffled surface is much preferred to a smooth one. In fact on a glassy calm water surface the pilot has essentially no knowledge of his or her height above the surface. In this case the pilot is trained to position the aircraft in a landing attitude, nose up, and apply sufficient power so that the sink rate is very low. In this manner the surface comes up to greet the aircraft when it pleases to surprise the pilot. One, of course, needs a long landing site for this procedure. Fortunately this is quite often the case when landing on water, except of course for small lakes.
By way of further background, for those aircraft utilizing microwave landing systems it is often the case that the co-pilot of the aircraft reads the microwave indication of altitude and calls the altitude of the aircraft out to the pilot during such time as the pilot lands. While the accuracy of this system is dependent upon the microwave radar altimeter, it is oftentimes an annoyance to the co-pilot to have to call out the altitude during the landing phase, since his duties occupy him otherwise. Of course, as mentioned above, the majority of aircraft do not have microwave altimeters and thus even if an experienced co-pilot were there to call out the height of the aircraft above the ground, his estimates would undoubtedly be dependent on his skill and therefore subject to error.
Moreover, while in the past there have been so called "talking depth sounders" such as exemplified in the U.S. Pat. Nos. 4,487,405; 4,621,348; 4,616,350 and 4,672,590, these patents relate to aquatic depth determining measurements which utilize sonar transducers singularly unsuitable for through-the-air range finding. However, Polaroid Corporation of Cambridge, Mass. has developed sonar range finding equipment for use in cameras. Unfortunately, the transducers utilized in the Polaroid systems are excessively subject to contamination and are therefore not usable in an environment in which hydrocarbons exist or in which either erosion or other particulate contaminants operate to degrade the output of sonar transducer. Moreover, the range of such transducers are generally limited to 75 feet at a maximum for reasons of their construction and for their application in cameras in which focusing at distances greater than 75 feet is not desired. Additionally, while through-the-air transducers have been utilized to control industrial processes in which the position of articles on a conveyer belt is monitored through short distance sonar ranging systems, these systems are operative only to tens of feet. Moreover they can be located in a controlled environment such that contamination of the transducer surfaces or faces can be controlled. Of course, these transducers can be periodically cleaned or operated in a clean environment to assure production line operation. Thus prior through-the-air sonar ranging systems do not operate over sufficient ranges for aircraft use and are not designed to operate in a "dirty" environment.